Referring first to FIG. 7, there is shown a connection end portion of an optical fiber 1 which is optically connected to a light emitting element. FIG. 7(a) is a perspective view of the optical fiber connection end portion. FIG. 7(b) is a cross-sectional view of the optical fiber connection end portion taken on an x-z plane indicated in FIG. 7(a). FIG. 7(c) is another cross-sectional view of the optical fiber connection end portion taken on a y-z plane indicated in FIG. 7(a). The optical fiber 1 has a circular shaped mode field 2. In addition, the optical fiber 1 has a connection end portion improved so as to be optically connected, at high optical coupling efficiency, to the emitting light having an elliptical shaped mode field from a semiconductor laser diode 3 (hereinafter, referred to as the semiconductor LD3). The connection end portion of the optical fiber 1 for optical connection to the semiconductor LD3 is formed into an approximately wedge-like shape and its endmost portion is a curved surface forming a lens. The optical fiber 1 is constructed so that it can collect rays of light emitted from the semiconductor LD3 with the lens.
Various types of optical fibers are available as an optical fiber 1, differing in mode field diameter w from each other. Likewise, there are various types of semiconductor LDs as a semiconductor LD3, differing in mode field diameter from each other. Generally, connection combinations of the type of optical fiber (as the optical fiber 1) connected to the type of semiconductor LD (as the semiconductor LD3) are determined according to specifications.
In the above-described condition in that the combination of the optical fiber 1 and the semiconductor LD3 by type is predetermined, a number of different technique types have been proposed with a view to improving the optical coupling efficiency between the optical fiber 1 and the semiconductor LD3. However, these prior art technologies all fail to provide satisfactory optical coupling efficiency.
For example, a most frequently requested connection combination is a connection combination of the optical fiber having a mode field diameter w of 6 .mu.m and the semiconductor LD3 having an emitting light wavelength of 980 nm. Although the connection end portion of the optical fiber 1 of FIG. 7 is improved so as to suit the connection combination of this specification, the optical coupling efficiency between the optical fiber 1 and the semiconductor LD3 has not yet reached satisfactory results.
The reason may be considered as follows. The optical coupling efficiency between the optical fiber 1 and the semiconductor LD3 is determined by the product of a optical coupling efficiency component in the direction of x and a optical coupling efficiency component in the direction of y (see FIG. 7(a)). The y-direction optical coupling efficiency component is determined by the curvature radius R of the lens formed at the endmost portion of the optical fiber 1 of FIG. 7(c), and the angle (wedge angle) .theta. defined between an inclined plane 4 forming the approximately wedge-shaped end portion of the optical fiber I and a core axis C. On the other hand, since optical connection with the semiconductor LD3 is a butt joint due to the x-z cross-section of the end portion of the optical fiber 1 is rectangular as shown in FIG. 7(b), the x-direction optical coupling efficiency is determined by the mode field diameter w of the optical fiber 1 and the type of the semiconductor LD3.
Generally, it is possible to determine the curvature radius R and the wedge angle .theta. at suitable values, regardless of the combination specification of the semiconductor LD3 and the optical fiber 1. This makes it easy to improve the y-direction optical coupling efficiency component. On the other hand, the mode field diameter w of the optical fiber 1 and the type of light emitting element are determined by the connection combination specifications, and, as an inevitable consequence, the x-direction optical coupling efficiency is determined by the connection combination specifications. Therefore, it is impossible to improve the x-direction optical coupling efficiency, and there is a limit to improving the total optical coupling efficiency between the optical fiber 1 and the semiconductor LD3. Under the present conditions, there have been no sufficiently satisfactory optical coupling efficiencies achieved.
The results of the examination made by the inventors of the present invention show that, as the circular mode field diameter w of the optical fiber 1 approaches the mode field diameter K1 of the elliptical mode field of the semiconductor LD3 extending in the major axis direction (i.e., in the ellipse's major axis direction), the x-direction optical coupling efficiency increases. However, for the case of the aforesaid combination of the semiconductor LD3 having an emitting light wavelength of 980 nm and the optical fiber 1 having a mode field diameter w of about 6 .mu.m (which combination is, as described above, a most frequently required combination), the elliptical mode field diameter K1 of the semiconductor LD3 having an emitting light wavelength of 980 nm extending in the ellipse's major axis direction is about 4 .mu.m, and the mode field diameter w of the optical fiber 1 is at a distance in value away from the mode field diameter K1 of the semiconductor LD3, having a value of about 6 .mu.m. Because of such a difference in value between K1 and w, the resulting x-direction optical coupling efficiency between the semiconductor LD3 and the optical fiber 1 is not very good. For such reasons, there have been strong demands for further improvement in the optical coupling efficiency between the optical fiber 1 and the semiconductor LD3.
In order to improve the optical coupling efficiency between the optical fiber 1 and the semiconductor LD3, the following may be considered. In addition to forming the y-z cross-section of the optical fiber's endmost portion into an approximately wedge-like shape, it is further arranged such that the optical fiber's endmost portion has an x-z cross-section of an approximately wedge-like shape. However, such arrangement will end up causing the endmost portion of the optical fiber 1 to have a complicated shape, thereby complicating the fabrication process of the optical fiber 1. Further, it is difficult to form the endmost portion of the optical fiber 1 into a desired shape with precision, therefore producing a problem in that the yield of the optical fiber 1 decreases drastically.
Further, if the mode field diameter K1 of the elliptical mode field of the semiconductor LD3 extending in the major axis direction (the ellipse's major axis direction) or the mode field diameter Ks thereof extending in the minor axis direction (the ellipse's minor axis direction) slightly differs, this causes the optical coupling efficiency between the optical fiber 1 and the semiconductor LD3 to vary considerably. This produces the problem of seriously limiting the type of the semiconductor LD3 capable of establishing optical connection at a high optical coupling efficiency.
The present invention was made in order to provide solutions to the above-described problems. Accordingly, a major object of the present invention is to provide an improved optical fiber having a circular mode field capable of optical connection to a light emitting element having an elliptical mode field at a high optical coupling efficiency, regardless of the connection combination specifications of the type of optical fiber and the type of light emitting element.